This Diet vs That Diet, and The Winner Is…

The Mediterranean Diet was recently compared directly to the Ketogenic Diet. 

And the winner is…well, more on that shortly.

For more than a decade now, I’ve had the privilege to serve among the judges for US News & World Report’s perennially anticipated “best diets” rankings. From that perch, I have annual reaffirmation of our popular fascination with that very issue: can we say what diet is best, and if so, for whom?

That has long been a matter of singular interest to me for a host of reasons. First, the importance of getting diet “right” cannot be overstated. Poor diet quality- and let’s be clear, there are many ways to eat badly and Americans seem committed to exploring them all- is the number one predictor of premature death and chronic disease in the U.S. and much of the world, year in, and year out. Second, I have a great antipathy for dogma and sanctimony, and more still for charlatanism. In that space where “my diet can beat all other diets!” is most often heard- fools, fanatics, and hucksters generally prevail. 

And finally, the answer to that question- “can we say what diet is best for health?”- has important implications for science, for policy, and for practice. If one specific diet were truly “best,” we would all have a “this way, or the highway” choice to make. If, instead, the answer involves more nuance, we are potentially empowered by options. No dietary pattern can benefit those unwilling to eat that way.

As a clinician, researcher, and writer- I have devoted decades to this question. My conclusion, in peer-reviewed papers, blogs, textbooks, and coffee table books has been the same. Yes, and no.

If the question means “is there some general way of eating that is reliably best for Homo sapiens?”- the answer is an irrefragable yes. How could it not be? 

For every other species on the planet, some suite of adaptations dictates the bounds of dietary prudence. We don’t need randomized trials to feel secure that dolphins are well off eating fish; sea otters, abalone; wildebeest, grass; and lions- wildebeest. How odd it would be if the kind of animal we are departs from all the rest of biology in this regard. It does not. 

So says a vast trove of science; so says sense; and so says the global consensus of experts. So says, too, the famously pithy wisdom of Michael Pollan: “eat food, not too much, mostly plants.” We don’t need individualized genomic profiling to know that’s right.

If, however, the question means “is there one, strictly defined, prescriptive diet that is incontrovertibly better than all others?”- the answer is almost as emphatically “no.” We have no basis to say, for instance, that the native dietary practices prevailing in one of the world’s Blue Zones is better than the alternative practices in another. Variants, all, on the theme of real “food, not too much, mostly plants”- all of those dietary patterns have produced comparable longevity and vitality for generations, with a patina of sustainability for good measure.

That last item is crucial. The plight of our beleaguered planetary health is arguably the single great medical imperative of our time, for failure to address this one may soon make all others moot. There are no healthy people on an uninhabitable planet.

No public health crisis ever encountered even hints at the menace of our extinction- but the collapse of planetary health and the loss of biodiversity do exactly that. Consequently, there can be no “best” diet anymore that isn’t as good for planet as it is for people. We can hope to be healthy, vital people on a healthy, vital planet- or we will have no hope of being healthy, vital people at all.

That, then, is the context for interpretations of “best diet.” There is a theme that is not debatable; there are variants on that theme that certainly are, which open portals to personalized nutrition. There is a need now for “best” to mean for people and planet alike, no others need apply.

To populate this well-defined expanse with reliable advances in our understanding requires that nutrition research be done right. Setting up studies to pit a winner chosen in advance against a straw man is all too easy, all too tempting for those with dogma in the fight, all too common, and all too likely to populate the teases and titillations of our news cycles. When studies ask a bad question, or weave bias into their bedrock, there is no methodology that can generate a good answer.

If we want good answers, we need good questions- and a commitment to fair competition. This is a surprisingly rare set of attributes and among the reasons nutrition research, and to some extent biomedical research in general, is so subject to criticism. Nutrition research can be done right, however, and no one puts that on display more reliably than Christopher Gardner of Stanford University.

I am tempted to call my colleague the “inimitable Prof. Gardner,” because his studies set a high bar others all too often fail to clear. But while Christopher is personally inimitable, his methods are just the opposite. They can, and should, be emulated by all seeking honest answers to important questions in nutrition.

Prof. Gardner’s latest effort is a recently-published, and much-covered study, comparing effects of a Mediterranean and Ketogenic diet on glycosylated hemoglobin, weight, and an array of biomarkers, in adults with diabetes or prediabetes. We may defer details of the study design to the paper itself, and simply note here that every effort was made to compare diets of comparable quality. This is no easy task, since the necessary restrictions of a ketogenic diet make high diet quality (measured objectively) elusive. Of note, the study did not report diet quality measures per se, nor were there data to indicate how reliably ketosis was achieved or sustained.

The findings were much as one might expect. Both dietary assignments were a step up from the “typical American diet,” and both were associated with improvement in the primary outcome, glycosylated hemoglobin. These improvements were statistically indistinguishable between groups, as was weight loss. There were some differential effects on biomarkers and nutrient intake levels, with most favoring the Mediterranean diet. Follow-up assessment of the study participants after the 12-week intervention showed that adherence to a Mediterranean diet was much greater than to a ketogenic diet.

In summary, then, dietary discipline of any kind is apt to confer metabolic benefits relative to the mayhem of the prevailing, American, “see food” diet (i.e., I see food, or junk masquerading as food, and I eat it…). Diet quality matters more than diet type, and when diets of comparably high quality are compared, outcomes are more alike than different. But, of comparable importance, the ketogenic diet is a contrived, restrictive diet that proves hard to maintain. Maintaining a Mediterranean diet is far easier. And finally, while the diets in this study both emphasized whole foods and plants, in prevailing practice the Mediterranean diet is apt to be the far friendlier of the two to planetary health.

Also of note, the long-term effects of the Mediterranean diet are known to be favorable. The long-term effects of a ketogenic diet are mostly unknown, because no populations are known to eat this way, and long-term study data are lacking. Reviews of the ketogenic diet have raised theoretical concerns, and noted the lack of data regarding long-term health outcomes.

All too often, the only clear winner in “this diet versus that diet” contests is spin, as the diet favored at the start predictably winds up looking better. Rare are the studies such as this new one that give us scrupulous methods and even-handed treatment. When research is done this way, the winner may or may not be any given diet- but is reliably us, as recipients of trustworthy, actionable information. Inevitably, the fundamentals of a healthful, sustainable dietary pattern are affirmed, as is the empowering range of variants on that common theme. So, too, as in this case, are the pros, and potential cons, of a choice we might consider.

Dietary quality prevails over both diet type, and diet hype. Nutrition research done right- thank you, Prof. Gardner!- points to the possibility of deriving pleasure from both better health and vitality, and from choosing food we actually want to eat.